A number of materials have already been used as projection screens for two-dimensional projections, but they are not entirely satisfactory. Coated materials in which a more or less large scattering effect influencing the lateral view characteristic attained by the type of coating is usually preferred for this purpose. The higher the degree of reflection of the projection screen, the lower the lateral view characteristic, and the stronger the undesirable scintillation or vice versa.
In the case of front projections a substantial disadvantage lies in that the viewers must almost exclusively be seated laterally of the picture screen to avoid disturbing the projection beams. An optically darker picture thus cannot be avoided when viewing the pictures. Of course, there exists the possibility of placing the projector above the heads of the viewers. However, this results in a distortion in trapezoidal form of the projected picture unless the picture screen is beveled rearwardly. However, this will again result in an optically darker picture. These disadvantages can be avoided by back projection with substantially brighter pictures. However, difficulties are frequently encountered in this case as well in that because of the structural conditions, the center portion of the projection screen appears brighter than the lateral surfaces (the so-called hot spot) and that as a result of the material, the filaments of the projector often are distinguishable. Furthermore, in many cases an undesired glair effect and a high scintillation are encountered.
For the back projection, plastic panes or rubber-like flexible materials are usually used. Heretofore attempts were made to overcome these disadvantages by special coatings, for example, wax coatings, but these methods were not really successful.
In projection screens for the three-dimensional projections there still exist substantial difficulties at the present time. The material used for this purpose heretofore, namely silver-coated or vapour-deposited linens have the decisive disadvantage of depolarization of incident light and of light passing through in addition to the disadvantages encountered in the screens for two-dimensional projections. Furthermore, the projected pictures are very bright at the center and relatively dark at the adges, and because of the depolarization or rotation of the polarized light, so-called "stereopictures" are frequently obtained.
Therefore, a projection screen having optimal optical properties must equally satisfy the following requirements:
1. high resolving power PA1 2. high degree of transmission, PA1 3. very good light distribution, PA1 4. wide picture reproduction range, i.e., color quality, PA1 5. almost no scintillation, PA1 6. possibility of use in daylight or artificial light and PA1 7. for three-dimensional projections, no depolarization or rotation of the polarized light.